As a need for media quality increases, the contrast of a video image becomes an important index of many electronic video devices. Same display devices include a manual video contrast adjustment: manually increasing the slope of the gray level mapping curve of certain fixed gray area to enhance the contrast of certain gray level. This manual adjustment inevitably results in the reduction of the contrast of other gray levels. When the gray level of video data distributes in the reduced gray levels, the contrast actually decreases, even the gray level would be combined.
Using several gray level mapping curves is often applied to dynamically adjust the contrast of video image according to the video image itself. First, the gray level mapping curve is determined according to the image's gray statistic data and scene detect signal gray level mapping is performed to the image for dynamically adjusting contrast. But this method has some defects: when there are tiny changes in gray distribution of scenes, the scene detect signal will switch in different modes in high frequency, which will leads to undesirable image flicker and using predetermined mapping curves limits the dynamic adjustment range and is costly because of the costs associated with storing the mapping grid.